Other studies have linked birth month to eventual intelligence, mental health, longevity and even wealth.

A closer look at previous studies, however, has uncovered a potential flaw: socioeconomic status seems to have a strong influence on the months that mums deliver their babies, and women who are poor, young, less educated and single tend to give birth in the months that have been linked with the worst consequences for babies.

For the new study, Professor Janet Currie and Dr Hannes Schwandt from Princeton University got around that problem by looking at siblings born to the same mothers.

"By focusing on births to the same mother, our work provides evidence that there are seasonal patterns in birth weight and gestation that are not entirely driven by the fact that women with different characteristics tend to give birth at different times," they write.

Currie and Schwandt collected data on 1.4 million births to nearly 650,000 mums in New York City, New Jersey and Pennsylvania. Then, they compared the outcomes of children conceived to identical mums but at different times of year.

When babies are conceived in May -- the last month of spring in the northern hemisphere -- they are 10 per cent more likely to be born prematurely compared to babies conceived in other months. Premature birth is usually linked to lower birth weights.

Flu link

The researchers couldn't definitely say why May was a riskier month to get pregnant, but they also found a direct link between infections with the influenza virus during pregnancy and early births.

And because the flu is most common in the northern hemisphere winter months of January and February, when May conceptions become full term, the influenza virus could explain why pregnancies that start in mid-spring may be more likely to end early.

"Because influenza is known to cause premature labour, these infants are at higher risk of short gestation," the researchers write.

But waiting until June, July or August -- summer in the northern hemisphere -- to get pregnant, seems to provide a boost to birth weight by about eight grams.

Women who conceive in those months gain around 360 grams more than women who conceive in January do, the researchers report. That larger pregnancy weight-gain is likely to contribute to bigger babies.

"Our results may have some implications for public policy, because they suggest that seasonal variations in nutrition matter for birth outcomes, even in rich countries, and that flu shots might be effective in fighting the seasonal deterioration in length of gestation," write the researchers.